The present invention concerns an anti-shrinkage additive for unsaturated polyester resins. The invention deals with a tri- or tetrafunctional polyether, condensation product of propylene oxide on a triol or tetrol, with this condensation product being acidified in such a manner that it possesses at least one terminal acidic functional group per elementary molecule.
The molding of objects based on unsaturated polyester resins is well known. It is also known that most conventional unsaturated polyester resins lead to volumetric shrinkage estimated at 8% to 10%, which causes numerous defects on the molded objects; such as shrinkage cracks, irregular dimensions, wrinkles, fissures, and poor surface appearance.
In order to alleviate these drawbacks, numerous additives have been proposed to be added to the unsaturated polyesters. For example, in French Pat. No. 1,530,817 or U.S. Pat. No. 3,925,300 thermoplastic polymers were added to certain types of unsaturated polyesters. Even though this addition makes it possible to obtain objects possessing a good surface appearance with very little shrinkage starting from a moist molding process, it is not effective in the case of mass moldings or leaf moldings. In German Pat. No. 2,354,716, it has been proposed to add to the polyester a vinyl ethylene acetate copolymer or vinyl ethylene propionate copolymer, but the results obtained are not very satisfactory industrially.
Not only are these techniques unsatisfactory, but they present the inconvenience of introducing considerable quantities of thermoplastic resins having formulations of generally elevated viscosity, requiring decreased fillers, which increases the price of the molded products.